No moreso than extension methods already did, I think. The only real concern is naming conflicts, which are generally an immediate compiler error that's easily resolved. Certainly a little more likely for something like an indexer, that doesn't HAVE a name, but what are the odds, really, of not only wanting to add an indexer to a type where it makes sense to have one and that type doesn't have one already, but also that there would be MULTIPLE different conflicting ways for that to make sense?

Personally, I think this has been a long time coming.